Higher mileage engines develop a varnish build up on the inside of the lifters in the area where they don't usually travel. Water will collapse the lifter, causing the inner to stick to the outer, due to that varnish.
One quikie fix is to give each rocker a couple or 3 good taps with a dead blow hammer. That may loosen it up.
Actual inspection would require pulling each lifter to look at it. If (IF!) a lifter or 2 is messed up like mine, be sure to inspect the cam lobe relative to the lifter.
This may or not be the problem though. It was a just suggestion as to what MIGHT be wrong. It's a job that's fairly involved, so, before getting into this, I'd make sure to check the other suggestions offered previously, such as an exhaust leak, any of the pulleys on the front of the engine, including the power steering pump/pulley, as that's a fairly common prob on these trucks for creating strange noises.
Make sure the plug wires are fully connected at each end. Sometimes wires can 'look' connected, but really aren't fully seated. Plugs tight? I've had ones back out on themselves occasionally.
Check for arcing in the dark, with the engine running. The underhood light can be turned off by bending it down with the hood up. Remember to bend it back up after.
Are the wires themselves in good shape? Old worn out ones will arc & that can actually be quite loud when they do.
Could be just the luck of the draw, though. Just seems that every time I work on my trucks, something unrelated seems to pop up right after I've just fixed something else.
You might even want to check to see if the flex-plate is cracked or broken. I've heard some sound like a bottom end knock. Quite loud! But I've also heard them sound kinda quiet, something like the sound in your vid.
Pretty hard to diagnose over the net, but those are my guesses!
Hopefully someone else can jump in here with some more/better ideas!
Hopefully I'll have time this weekend to redo the gaskets. 
